The present invention relates to the monitoring of errors in video signals, and more particularly to the detection of frozen fields in video that has been encoded according to an analog television standard.
Companies that distribute video over compressed pathways, such as satellite or digital cable distributors, have hundreds of channels that need to be monitored for defects. These companies need an automated method of monitoring for such defects. One of the most common of these errors is frozen fields, in which the video ceases to change, or is black, due to equipment failure. The current instrument that is used to detect such an error is the Tektronix PQM300 Picture Quality Monitor manufactured by Tektronix, Inc. of Beaverton, Oreg., United States of America which uses a single-ended measurement algorithm described in U.S. patent application Ser. No. 09/152,495 entitled xe2x80x9cPicture Quality Measurement Using Blockinessxe2x80x9d and filed Sept. 10, 1998. However this single-ended blockiness measurement does not decide whether the blockiness being detected is caused by unacceptable CODEC deformation of the image or is the natural result of blockiness that exists within the video. A method for detecting blockiness caused by DCT-based CODECs is described in U.S. patent application Ser. No. 09/518,430 entitled xe2x80x9cBlockiness Period Detection of DCT-Based CODECsxe2x80x9d filed Mar. 3, 2000.
However if the frozen video is at any point in analog encoded format, such as NTSC or PAL, the chrominance variations inherent in these standards cause even visually identical fields to be significantly different on a pixel by pixel basis. Enough so that a measurement that determines the difference between two otherwise identical fields while not yielding false positives on similar, but non-identical, fields is extremely computationally expensive. This variance remains even if the video stream is subsequently converted back into a digital format. For example comparing field one of one frame to field one of the next frame results in indications of differences between the fields due to artifacts of the analog encoding process that are 180xc2x0 out of phase due to the color phase clocking, even though the fields are otherwise identicalxe2x80x94frozen.
What is desired is a means of detecting frozen fields in video data that has been at some point analog encoded.
Accordingly the present invention provides a frozen field detection technique for previously analog encoded video data by obtaining a difference between each field and its nth subsequent field where the color phasing of the analog encoding standard is identical. Then when the difference is essentially zero, a frozen field indicator is initiated to indicate the presence of frozen fields in the video signal.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing figures.